The present invention relates to an optical gas densitometer for continuously and rapidly measuring the density of a specific gas by utilizing an absorption band inherent in the infrared wavelength region of that gas molecule.
It is generally desirable that the optical gas densitometer have a high accuracy rate. Three prior optical gas densitometer systems will be explained with reference to FIGS. 3-5 of the accompanying drawings.
FIG. 3 shows a schematic view of a conventional gas analyzer 1 disposed at the outside of a channel 10 of a gas to be measured. The gas analyzer 1 samples the gas from a gas sampling tube 9 inserted in the channel 10, by using suction means, for example, a pump 4. The analyzer 1 carries out pre-treatment such as dehumidification in a drain separator 2, a drain pump 3 and a dehumidifier 5, and the analyzer 1 removes dust or mist in a filter 6. The gas to be measured is then introduced into an analyzer meter 8 for analysis. Reference numeral 7 denotes a sampling gas flowmeter for sampling the measured gas.
This known method, however, is disadvantageous in that there is a substantial delay in time for the meter to actually indicate the density of the gas. This is caused by the extended distance from the sampling point at the gas sampling tube 9 to the gas analyzer 1, the flow rate of the sampled gas, and the pre-treatment and filter section. As a result, this analyzer cannot be put to practical use in a case, for example, of a combustion control in a boiler where fast response is required. Moreover, to reduce the time delay by increasing the suction amount of the sampled gas would only promote maintenance problems such as contaminations in the gas sampling tube 9 or the pre-treatment section.
FIG. 4 shows a schematic structural view of a gas analyzer utilizing a portion of the flow channel of the measured gas as an optical path. An optical source portion 11 and a detection section 12 are oppositely disposed on the diametrical direction of the flow channel 10 of the measured gas, for example flue 10.
This known analyzer, however, has the following defect. It is important but difficult to align the optical axis of the optical source portion 11 with that of the detection section 12. Since the flue 10 generally has a diameter of several meters, a slight dimensional change due to heat distortion or the like in the optical source portion 11 and the detection section 12 may cause a large deviation between their optical axis that would worsen the stability needed for alignment. Further, additional materials are required for installing the optical source portion 11 and the detection section 12. The disposition of a blower is required to prevent the contamination in the light permeating windows for the optical source portion 11 and the detection section 12 in direct contact with the measured gas. Moreover, since it is impossible to fill the flue 10 as the measuring optical path with a standard calibration gas, calibration by the use of the standard calibration gas which is of highest accuracy as the ordinary gage calibration method cannot be employed.
FIG. 5 shows a schematic view of a gas analyzer in which a space between a pipe 14 and a cylindrical filter 15 attached at the top end of the pipe is used as a measuring optical path. A gas analyzer 13 has a front end-closed cylindrical filter 15 attached at the front end of a pipe 14, and a reflection mirror 16 disposed to the inside of the front end of the cylindrical filter 15, in which light is returned by the reflection mirror 16 through a half mirror 17 to introduce a portion of the reflection light to a detector 18. Reference 19 is an optical source, 20 is a chopper for the optical source 19, and 21 is a lens.
This known method, however, is also disadvantageous in that in a flue 10 of a large diameter the ratio of the optical length contributing to the measurement relative to the entire optical length L is small to the case of measuring the gas density at the middle portion of the flue 10. Moreover, the loss of the optical amount is large due to the reflection of the half mirror 17. Another drawback is that the light source 19 requires a large amount of power.